This invention relates to a thin film semiconductor device used as a driving substrate of an active matrix liquid crystal display panel or the like. More particularly, it relates to a thin film semiconductor device using ordinary glass as a substrate and made by low temperature processes. Still more particularly, it relates to technology for preventing adverse affects of alkali metals contained in the glass.
A thin film semiconductor device is a device wherein a thin film transistor is formed on an insulating substrate, and because they are ideal for example for driving substrates of active matrix liquid crystal display panels their development has been being advanced vigorously in recent years. Particularly when using a thin film semiconductor device in a large-area liquid crystal display panel, it is essential to reduce the cost of the insulating substrate, and glass substrates are being employed instead of the relatively high quality quartz substrates used in the past. When a glass substrate is used, because its heat resistance is relatively low, the thin film transistors must be formed by low temperature processes of below 600xc2x0 C. Now, as a semiconductor thin film constituting active layers of the thin film transistors, amorphous silicon and polycrystalline silicon have been used. However, from the point of view of the operating characteristics of the thin film transistors, polycrystalline silicon is superior to amorphous silicon. For this reason, the development of polycrystalline silicon thin film transistors made by low temperature processes has been being advanced in recent years.
When polycrystalline silicon is used as an active layer of a thin film transistor formed on a glass substrate, contamination caused by alkali metals such as sodium (Na) contained in the glass substrate has been a problem. Polycrystalline silicon is more sensitive to alkali metal contamination than amorphous silicon, and with polycrystalline silicon such contamination has an adverse influence on the operating characteristics and reliability of the thin film transistor. For example, if an alkali metal diffuses into the gate insulating film of a thin film transistor the device characteristics change. When at a high temperature a bias is applied and an operating test is carried out, the device characteristics change greatly because alkali metal in the gate insulating film moves and polarizes and concentrates in localities. Consequently, when thin film transistors have been formed on a glass substrate, the practice of forming in advance as a base layer a silicon nitride film (SiNx) or a phosphorus-containing glass (PSG) as a buffer layer has been carried out. By this buffer layer being interposed, the vertical diffusion of alkali metal from the glass substrate toward the gate insulating film is suppressed and contamination of the gate insulating film is prevented.
However, it has become clear that just preventing vertical movement of alkali metal is not sufficient. That is, horizontal diffusion of alkali metal included in the glass substrate occurs due to bias of the driving voltage impressed on the thin film transistor, and alkali metal ions polarize and concentrate locally. An electric field is created by local polarization of charges of alkali metal ions, and this similarly has an adverse affect on the operating characteristics of the thin film transistor. It has become clear that as a result of this the threshold voltage and the leak current of the thin film transistor undergo fluctuations. It is extremely difficult to prevent this horizontal movement of alkali metal in the glass substrate. For this reason, for example in U.S. Pat. No. 5,349,456 a method for removing Na from a glass substrate is disclosed. However, this method is not always practical because it greatly diminishes the merit of using a low cost glass substrate.
Accordingly, it is an object of the invention to solve the problem described above and provide a thin film semiconductor device comprising a thin film transistor formed on a glass substrate wherein an electric field arising as a result of horizontal diffusion of alkali metal in the glass substrate is effectively and cheaply prevented from adversely affecting the operating characteristics of the thin film transistor.
To achieve the above-mentioned object and other objects, a thin film semiconductor device according to the invention comprises as a basic construction a glass substrate containing an alkali metal, a buffer layer covering the surface of the glass substrate and a thin film transistor formed on the buffer layer with a polycrystalline semiconductor thin film as an active layer. As a characterizing feature of the invention, the buffer layer includes at least a silicon nitride film and protects the thin film transistor from alkali metal contamination and has a thickness such that it can shield the thin film transistor from an electric field created by localized alkali metal ions. In one form of the invention, the thin film transistor has a bottom gate structure wherein a gate electrode, a gate insulating film and a semiconductor thin film are superimposed in order from the bottom. In this case, the semiconductor thin film has a channel region located directly above the gate electrode, high concentration impurity regions located on either side of the channel region and low concentration impurity regions interposed between the channel region and the high concentration impurity regions. The low concentration impurity regions are shielded from an electric field forming in the glass substrate by the buffer layer. Preferably, the gate insulating film includes a silicon nitride layer and is superimposed with the buffer layer and the two synergetically protect and shield the thin film transistor. In this case, the total thickness of the mutually superposed gate insulating film and buffer layer is over 200 nm. The buffer layer is preferably a two-layer structure made up of a silicon nitride film and a silicon oxide film. In a specific construction, a pixel electrode is formed connected to at least a part of the thin film transistor and the thin film semiconductor device can be used in a driving substrate of an active matrix display panel.
In the invention, a buffer layer is interposed between a glass substrate and a thin film transistor. This buffer layer includes at least a silicon nitride film, and blocks vertical movement of alkali metal and thereby suppresses contamination of the gate insulating film. The silicon nitride film has a fine composition, and by making its thickness above 20 nm it is possible to substantially completely prevent Na and the like from passing through it. Also, in addition to the silicon nitride film this buffer layer includes for example a silicon oxide film and has a two-layer structure. Because film stresses in the silicon oxide film are smaller than in the silicon nitride film it is possible to make the thickness of the buffer layer as a whole large and thereby electrically separate the thin film transistor from the glass substrate. By making the thickness of the buffer layer at least 100 nm it is possible to electrically shield the thin film transistor from the glass substrate. Therefore, it is possible to shield the thin film transistor from adverse affects of an electric field formed as a result of horizontal diffusion of alkali metal inside the glass substrate. As a result, it becomes possible to maintain the reliability and operating characteristics of the thin film transistor even when a glass substrate containing alkali metal is used.